A bone anchor assembly, comprising: a bone anchor (100) having a proximal head (104) and a coupling portion (106) to the distal bone； and a receiving member (108) having a proximal end (110) having a first hole (116), a distal end (112) opposite the proximal end and having a second hole (120), and a recess (114 ) to receive a spinal fixation element in communication with the first hole, the second hole being sized to receive the bone anchor head, the receiving member having a cross-section with a substantially U-shaped defined by two legs (124A, 124B) separated by the recess, each leg being free at the proximal end of the receiving member, in which a first groove (130A) is formed on an outer surface of the proximal end of a first (124A) of the legs and is formed a second groove (130B) on an outer surface of the proximal end of a second (124B) of the legs, the first and second grooves configured to receive a portion (72A) of an instrument (10) to be connected so that it can be release the instrument to the assembly, the first groove having a proximal surface (134) and a distal surface (132) opposite the proximal surface and the second groove having a proximal surface (134) and a distal surface (132) opposite the surface proximal, characterized in that the proximal surface of the first groove and the proximal surface of the second groove are each oriented at an angle between approximately 5 degrees and approximately 30 degrees with respect to a line (142) that is oriented perpendicularly with respect to a longitudinal axis (118) of the receiving member, such that it inhibits the separation of the instrument portion from the groove in a radial direction.Un conjunto para anclaje de huesos, que comprende: un anclaje de huesos (100) que tiene una cabeza proximal (104) y una porción (106) de acoplamiento al hueso distal； y un miembro (108) de recepción que tiene un extremo proximal (110) que tiene un primer orificio (116), un extremo distal (112) opuesto al extremo proxima
